PORTLAND, Maine — Supreme court justices on Thursday questioned whether utilities regulators shirked their responsibility for ensuring the safety of “smart meters” when they allowed customers to opt out without giving assurances to other customers who have them.

Opponents of Central Maine Power’s wireless-enabled electric meters say the devices emit radio frequency radiation that risks health and are an invasion of privacy because of the detailed data they collect. The Maine Public Utilities Commission allows customers to opt out of using them if they pay $12 a month, but the opponents say customers shouldn’t have to pay more if they don’t want the devices.

Central Maine Power contends the 600,000-plus smart meters that have been installed are safe, cut energy use and allow utilities to quickly pinpoint problems during power outages.

Bruce A. McGlauflin, a lawyer for smart meter foes, said elimination of the $12-a-month-fee would alleviate some of the opponents’ concerns, but not others including the safety problems.

Story continues below advertisement.

On Thursday, skeptical justices questioned a lawyer for the PUC about why the agency failed to meet its statutory obligation of addressing the safety of smart meters.

Jordan D. McColman, lawyer for the PUC, responded that other agencies with greater expertise had found no credible studies that demonstrated harm and that the panel felt the opt-out option addressed lingering concerns for customers.

But justices repeatedly pressed him about why the commission didn’t put that in writing.

Federal stimulus dollars funded roughly half the $200 million cost of the smart meter project. All but a couple thousand of them have been installed in CMP’s coverage area and the remainder should be installed by next month, said company spokesman John Carroll.